The present invention relates to a method for fabricating nano-size dots and, more particularly, to a method for fabricating erbium-doped silicon nano-size dots using the pulsed laser deposition in combination with the ultra high vacuum chemical etching reaction. Erbium-doped silicon nano-size dots thus produced have high purity, density and luminous efficiency.
In general, although silicon (Si) is unusable as an optical material because it has an indirect band gap, using the nano-sizing quantum effect in combination with luminescent material doping technology permits the realization of a silicon-based electrooptic method. Namely, it is necessary to have a doping method that enables efficient luminescence and a method for producing nanometer-scale particles.
Recently, many attempts have been made to produce erbium-doped silicon nano-size dots by chemical vapor deposition (CVD), sputtering, pulse laser deposition (PLD) techniques, and the like. Such erbium-doped silicon nano-size dots are used as a novel material for light sources having a wavelength of 1540 nm, at which the silica optical fiber has a very low absorbance. However, there is a need of using the pulsed laser deposition (PLD) in combination with the ultra high vacuum chemical etching reaction in order to maximize the erbium doping concentration and to have nanometer-scale size, both of which are essential to achieve a high luminescent efficiency.
The pulsed laser deposition (PLD) allows an increase in the erbium concentration by 10 to 100 times relative to the known chemical vapor desposition (CVD) or sputtering, and the ultra high vacuum chemical etching reaction enables production of a nano-scale structure under restricted conditions that prevent potential introduction of impurities, thus enhancing the electrooptic efficiency.
In a first example of the prior art for fabricating silicon thin films, multiple targets are mounted in a process chamber to produce multi-element thin films by pulsed laser deposition (PLD) (See, Douglas N. Mashburn et al., xe2x80x9cMultiple target laser ablation systemxe2x80x9d, Jan. 5, 1996, U.S. Pat. No. 5,483,037). This method deposits thin films using the pulsed laser deposition, in which multiple targets are mounted in a high vacuum process chamber and excimer laser beams are alternately irradiated on the individual targets to vaporize the components of each target to be deposited on a film, which is placed at a position to face the targets. That is, the intensity of the laser beam irradiated on the individual target can be regulated so as to control the amount of the element to be deposited on the film, thereby facilitating deposition of multi-element films such as YBCO.
In a second example of the prior art for pulsed laser deposition (PLD), a laser beam is split by a beam splitting means and the split beams are simultaneously irradiated on the same point of a target from different directions via a mirror so as to uniformly assign the irradiation time of the laser beam on the surface of the target (See, Douglas N. Mashburn et al., xe2x80x9cDual beam optical system for pulsed laser ablation film depositionxe2x80x9d, Sep. 24, 1996, U.S. Pat. No. 5,558,788). This method maintains the efficiency of the laser ablation on the surface of the target for a long time in uniformly depositing the thin films to prevent texturing on the surface of the target and places the plume to face the target in order to uniformly deposit the thin films on the target.
In a third example of the prior art for fabricating erbium-doped silicon thin films by pulsed laser deposition (PLD), a KrF laser beam having a wavelength of 248 nm is irradiated on a target which is prepared by mixing Er2O3 powder with silicon powder for laser ablation (See, Shuji Komuro et al., xe2x80x9cRoom temperature luminescence from erbium-doped silicon thin films prepared by laser ablationxe2x80x9d, Applied Physics Letters, Vol. 69, No. 25, p3896-3898, Dec. 26, 1996).
In a fourth example of the prior art, a method for fabricating silicon nano-size crystals by pulsed laser ablation under the inert gas atmosphere is disclosed, which adopts a process for fabricating well-dispersed silicon nano-size crystals under controlled laser conditions and examines the effect of the pressure of the inert gas on the transition of amorphous silicon thin films to nano-size crystals (See, Nobuyasu Szuki et al., xe2x80x9cStructure and optical properties of silicon nanocrystallities prepared by pulsed laser ablation in inert background gasxe2x80x9d. Applied Physics Letters, vol. 76, No. 11, p1389-1391, Mar. 13, 2000).
However, as disclosed in the third example of the prior art, the use of a target prepared by firing a mixture of Er2O3 powder and silicon powder at a high temperature in the fabrication of erbium-doped silicon thin films by pulsed laser deposition (PLD) contains a possibility of contamination with impurities and requires a plurality of targets that are different in the doping concentration.
It is an object of the present invention to solve the problems with the prior art and to provide an apparatus and method for fabricating silicon thin films, in which a highly pure solid target is vaporized in a ultra high vacuum process chamber so as to prevent any potential introduction of impurities and the intensity of the laser beam is regulated to vary the doping concentration.
It is another object of the present invention to provide a method for fabricating erbium-doped silicon nano-size dots with high electroluminescence efficiency and high purity using a surface chemical reaction on the erbium-doped silicon thin films.
In one aspect of the present invention, there is provided an apparatus for fabricating silicon thin films, which uses laser ablation, the apparatus including: a silicon substrate rotatably mounted in a process chamber maintaining a ultra high vacuum; pulsed light source means mounted outside the process chamber for emitting a pulsed light beam; target rotating means mounted in the process chamber for rotating a plurality of targets mounted therein, the targets being made of a different material; light beam splitting means for splitting the pulsed light beam into double light beams of appropriate intensities; light beam intensity regulating means for regulating the intensity of the double light beams. The targets are mounted to face the silicon substrate so as to uniformly overlap the vaporization products of the targets generated by irradiating the double light beams on the silicon and erbium targets.
In another aspect of the present invention, there is provided a method for fabricating silicon nano-size dots, the method including the steps of: forming erbium-doped silicon thin films on a silicon substrate by pulsed laser ablation under ultra high vacuum; nitrifying the surface of the silicon thin films to form a plurality of silicon nitride islands; selectively etching the silicon thin films which is not capped by silicon nitride by way of oxygen-induced etching to form erbium-doped silicon nano-size dots.